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Effect of Genetic Differences Among Herds on Accuracy of Selection and Expected Genetic Change¹

Department of Animal Sciences, Iowa State University, Ames 50011

ABSTRACT

Improperly accounting for genetic differences among herds affects the accuracy of estimates of breeding values. The loss is expected genetic change resulting from this lower accuracy was quantified. Expected genetic change was calculated by the sequential selection theory and was expressed in terms of accuracy of estimates of sire and cow breeding values. Simple linear models were used to explain milk production. Selection index theory was used to calculate accuracy as the expected correlation among indices and breeding values. The expected genetic change was calculated and plotted for variations in accuracy of estimates of breeding values. Expected genetic change increased from .4 to .6 kg and 1.1 to 1.2 kg of milk per cow per year for each .01 increase in accuracy of estimates of breeding values of cows and sires, respectively. Evaluation of the potential for genetic differences among herds indicated that the maximum improvement in the expected genetic progress would be around 10 kg of milk per cow per year.

¹ Journal Paper No. J-8601 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 1053, in collaboration with the North Central Region project NC-2, Improvement of Dairy Cattle Through Breeding.